Regression tests for import and export of the spent-addresses-db

[jakubcech]

Description

We want a regression test for both the spent-addresses-db import and export scenarios mapped in #1450 and 1462.

Motivation

Ensure the IXI scenarios aren't broken.

Issues / Scenarios

Export

1. I take a node with a spent-addresses-db
2. I use IXI to export the DB
3. A spentaddresses.txt file is created according to #1450

Import

1. I run a node
2. I take the export from multiple nodes created by the export scenario
3. I run an IXI specifying the files to import
4. The spent-addresses-db on the node from which I ran the IXI consumes the files and its spent-addressses-db is populated by the addresses in there.

[GalRogozinski]

We currently have the iri ls-sa merger tool. We need to think after the release of 1.9.0 how we want to test it